1. Field of the Invention
This invention relates to a post-engine emissions treatment device, and specifically to an integrated muffler and catalytic converter designed to heat and catalytically treat engine emissions.
2. Background of the Invention
The contribution of engine emissions to environmental pollution is undeniable. A myriad of chemical compounds are produced as a result of the combustion process, the reactions of which are depicted in unbalanced Equations I (excess air) and II (excess fuel) below: ##STR1## wherein (HC).sub.x represents fuel and (O.sub.2 +3.76 N.sub.2) represents the relative stoichiometric amounts of oxygen and nitrogen in air.
The three major targets of emissions clean up are hydrocarbons (HC), carbon monoxide (CO) and, more recently, nitrous oxides (NO.sub.x). Excess HC and CO in emission streams result from incomplete combustion, or too little air in the reaction zone. Some exhaust designs, among them the one disclosed in U.S. Pat. No. 1,824,078, have attempted to facilitate combustion of effluent constituents by providing additional air and heat to a reaction zone. However, excess air, combined with the higher operating temperatures associated with today's smaller engines, causes the formation of nitrous oxides.
In theory, the oxidation of any carbon monoxide and hydrocarbons that manage to elude the engine's combustion chamber should be comparatively simple with the addition of air to the still hot exhaust stream; but efforts to provide additional air have resulted in an adverse reduction of temperature at the reaction zone. Furthermore, while notable decreases in hydrocarbon and carbon monoxide concentrations in exhaust gases occur when effluent temperatures exceed 800.degree. C., exhaust gas temperatures rarely reach that level. Instead, the exhaust manifold attains temperatures of only 325 to 750.degree. C. during urban cycle driving speeds of between 0 and 56 miles per hour (90 kilometers per hour). Obviously, these temperatures are lower when the exhaust reaches the catalyst/muffler downstream, and lower still when ambient temperatures are low.
Catalysts are used in low temperature exhaust streams to facilitate the combustion reactions depicted in Equations I and II, supra. There are generally two types of catalysts: reduction catalysts, which break down nitrous oxides into their component nitrogen and oxygen molecules, and oxidative catalysts, which are used to oxidize HCs and CO to H.sub.2 O and CO.sub.2, respectively.
Many oxidative catalyst configurations require a lean fuel-exhaust mixture in combination with either an air pump to facilitate complete burning, or an aspirator preceding the catalyst to siphon off any residual, unburned fuel to an activated carbon trap. Without such air pumps or aspirators, HC and CO conversion efficiencies peak at only 65 and 45 percent, respectively, after use in a car that has been driven approximately 50,000 miles. Air pump and aspirator usage increases these HC and CO conversion efficiencies to 80 and 75 percent, respectively.
As noted above, an increase in combustion air for more complete burning of HCs and CO increases the likelihood for NO.sub.x formation, as NO.sub.x formation is the direct result of the following oxidation reaction: EQU N.sub.2 +O.sub.2 =2NO.sub.x
To address the clean-up of all three emission constituents, three-way catalysts are typically employed to first reduce the nitrous oxides and then oxidize the HCs and the CO. An inherent requirement in this type of configuration is a low oxygen environment immediately prior to treatment with a reducing catalyst, and a high oxygen, high temperature environment further downstream to facilitate CO and HC oxidation with an oxidation catalyst. To effect this reaction sequence, a three-way catalyst is first employed for initial clean up of exhaust, and then another catalyst, downstream from the three-way catalyst, provides additional treatment. In these configurations, air pumps are necessary to facilitate oxidation reactions.
Another drawback to current designs is their primary reliance only on heat from the exhaust to initiate the catalytic process. As automobiles typically do not attain the minimum 250.degree. C. necessary for catalysts to begin their reduction and oxidation (redox) functions for several minutes after ignition, and considerably longer when ambient temperatures are low, high levels of untreated exhaust constituents are liberated. This problem has prompted the passage of the Clean Air Act Amendments of 1990, 42 U.S.C. Sect 7521 (j), which stipulate more stringent emission controls of CO at 20.degree. F. by 1994.
A need exists in the art for an economical, post-engine emissions clean up device for a myriad of applications, including but not limited to mobile and stationary engines powered by gasoline, ethanol, diesel fuel, LP gas, compressed natural gas, or combinations thereof. This device would also serve to attenuate exhaust noise to a level below legally acceptable noise levels. The integrated device would contain redox chambers by providing a low oxygen reaction zone or stage for NO.sub.x reduction and a high oxygen reaction zone or stage for HC and CO oxidation. Also contained in the integrated process would be a preheater to initiate the catalytic process during the first few, critical minutes after initiation of combustion. To minimize cost and maintenance, the design of the device would allow for easy replacement of catalysts and would not require an air pump.